When an audio signal is compressed by an encoder, such as by a code excited linear prediction (CELP) type encoder the additive noise that may be present in the background when the audio signal is recorded, will be processed with the speech signal. This noise component is not desirable because it contributes to degrade the speech quality when a decoder processes the compressed audio signal in order to build a replica of the original signal. In this context, reducing the noise component in the signal while keeping only the periodic component of the speech signal would greatly enhance the speech quality.
At present, one of the techniques used for noise reduction is called center-clipping. With this technique, distortions may be introduced into the speech signal due to a disturbance in the short-term correlation properties, or, viewed in the frequency domain, distortions in successive short-term spectra may result. In contrast, the LPC residual is spectrum flattened and minor nonlinear operations do not introduce significant changes in the spectral shapes.
Thus, there exists a need in the industry to provide a method and an apparatus for enhancing speech quality by reducing noise that may be present in the speech signal.